Electrical energy is often generated, transmitted and distributed by alternating-current (AC) in poly-phase arrangements, such as three-phase arrangements with phase-lines L1, L2 and L3. In energy networks, power lines transmit electrical energy between nodes, such as power stations or sub-stations.
There are many types of power lines. Some of them transmit energy at voltages of 220 kV (kilo volt, or more) and with an electrical power in the magnitude of several 100 MVA (mega volt ampere, apparent power).
AC periodically changes between positive and negative peaks, but the peaks for voltage and current are not in synch. For a fraction of a period, the current is lagging behind. This difference in time is usually represented by the so-called phase angle displacement ϕ (or phase shift ϕ.